The science of life : fully illustrated in tone and line and including many diagrams

THE HARMONY AND DIRECTION OF THE BODY-MACHINE

and he cannot get such an accurate idea of the direction from which sounds are coming. The tube leading from this reduced ear-trumpet to the ear-drum is provided with hairs and wax-secreting cells that have a filtering and protecting function similar to that of the hairs and mucussecreting cells of the nose.

The middle ear is a narrow chamber filled with air and communicating with the mouth by means of a duct, the Eustachian iube. The walls of this tube are usually pressed flat together, but when the pressure in the middle ear becomes too different from that outside, they are forced open and a little air passes one way or the other. This causes the click in the ear which we experience after ascending or descending rapidly in a lift or mountain railway. The walls of the middle ear are strong and bony, except in three places. The first and largest of these is

the ear-drum, and in the second and third,

the fenestra ovalis and fenestra rotunda (oval window and round window), there are even more delicate membranes which separate middle and inner ear. A chain of three minute bones, prettily jointed together, runs from the ear-drum to the fenestra ovalis; because of their curious shapes these bones are called the hammer, the anvil, and the stirrup.

Sound waves, travelling down the outer ear, strike the ear-drum and make it vibrate ; by means of the chain of bones, the vibrations of the drum are transmitted to the fenestra ovalis. This elaborate device is partly protective, for the hammer is so jointed to the anvil that if a violent jar occurs, such as a box on the ear, the two are disengaged and the shock is not passed on to the delicate inner structures. But

Ear drum

there is an even more important reason. The inner ear is filled with a watery fluid, and the properties of sound waves travelling in water are very different from those of sound waves travelling in air. The bones act as levers and reduce the amplitude of the vibrations, but at the same time, by concentrating the energy of the vibrating ear-drum on to a window only one-twentieth of its size, they make the sound pulses more vigorous. They are a means of getting over the difficulty of transmitting soundwaves from air to water.

The inner ear is an elaborate labyrinth of passages, embedded in bone and filled with

Cochlea

Eustachian tube

Fig. 44. A section through the ear to illustrate the mechanism of hearing.

a lymph-like fluid; as we have seen, this fluid is made to vibrate when sounds fall on the outer ear.

A part only of the inner ear is concerned with hearing. We shall see later what the rest is for. The part that concerns us at present is a narrow, tapering tube, about an inch long, which is coiled into a dwindling spiral like the shell of a snail and is therefore called the cochlea. This cochlea is divided into three compartments by a partition which runs along its whole length. On this partition the sense-cells are situated. The sense-cells are represented in Figure 46 ; they bear stiff, hair-like projections on their